1. Field of the Invention
The present invention relates to a DC—DC converter for down converting an input DC voltage to a fixed value DC voltage, that is, a ripple detection self-oscillating step-down converter and a converter device having a plurality of connected stand-alone converters.
2. Description of the Related Art
Presently, low voltage and large current DC—DC converters are required due to the demand for power supply circuits in computers, etc. PWM controlled converters and ripple detection type self-oscillators (hereinafter simply referred to as ripple converters) representing such DC—DC converters are used. Since the response to load change is excellent, ripple converters, used since before the later developed PWM controlled converters, are being paid attention to once again.
FIG. 12 is a circuit diagram showing the basic circuit of a ripple converter.
As shown in FIG. 12, the ripple converter contains a PNP transistor Tr1 as a switching element and an inductor L01 connected in series between an input terminal 3, to which an input voltage Vin is input, and an output terminal 4 from which an output voltage Vout is output. The ripple converter also contains a flywheel diode D01 connected between the ground (and the ground terminal 5) and the connection point of the PNP transistor Tr1 and the inductor L01. Furthermore, a voltage in accordance with the output voltage Vout is input to the non-inverting input terminal, a reference voltage Vo is input to the inverting input terminal, and a comparator 10 outputting a switching control signal to the PNP transistor Tr1 is provided.
In such a ripple converter, when the PNP transistor Tr1 is in the off state and the output voltage Vout becomes lower than the reference voltage Vo, a Low signal is output from the comparator and input to the base of the PNP transistor Tr1, and then, the PNP transistor Tr1 is turned on. Then, when the PNP transistor Tr1 is turned on, the output voltage Vout increases. On the other hand, when the PNP transistor Tr1 is in the on state and the output voltage Vout becomes higher than the reference voltage Vo, a High signal is output from the comparator and input to the base of the PNP transistor Tr1, and then, the PNP transistor Tr1 is turned off. Then, since the PNP transistor Tr1 is turned off, the output voltage Vout decreases. By repeating such a control, the output voltage Vout goes up and down around a voltage close to the reference voltage Vo and an output voltage Vout substantially equal to the reference voltage Vo is obtained.
FIG. 13 shows the output voltage waveform of a related ripple converter.
As shown in FIG. 13, the output voltage Vout becomes a triangular waveform having a ripple in which the voltage (amplitude) goes up and down from a set voltage Vset, as a reference, on the basis of the reference voltage Vo. Then, in normal operation, the average voltage of the output voltage Vout is substantially in the middle between the maximum voltage and the minimum voltage in the waveform.
As a practical circuit of such a ripple converter, Japanese Unexamined Patent Application Publication No. 9-51672 discloses a ripple converter in which a switching element of a P-type FET and a choke coil are connected between an input terminal and an output terminal and a diode is connected between the ground potential and the connection point of the p-type FET and the choke coil. Furthermore, the ripple converter contains a comparator in which a voltage in accordance with the output voltage is input to the inverting input terminal and a reference voltage is input to the non-inverting input terminal and a driver IC outputting a switching control signal to the p-type FET in accordance with the output voltage of the comparator. Then, in this ripple converter, a desired output voltage is obtained from a fixed input voltage in such a way that the output voltage is compared with the reference voltage and the P-type FET is switched on the basis of the comparative result.
Now, in the related ripple converter in which the output voltage Vout is controlled so as to be constant, when the switching control is continuously performed, the duty factor, which is the ratio of the on time to the total of the on time and off time of a switching element such as an FET, etc., is dependent on the input voltage Vin. Or in a ripple converter in which the input voltage Vin is constant and the output voltage Vout changes according to the set condition, the duty factor is dependent on the output voltage Vout.
In the related ripple converter, there has been a problem in that, when the duty factor changes, the output voltage varies. Hereinafter, the principle is described.
FIG. 14 shows the waveform of the instantaneous value of the output voltage Vout, the average value Vavg, the set voltage Vset (dependent on the reference voltage Vo), and the on and off state of the switching element in the case where the input voltage is high. Furthermore, FIG. 15 shows the waveform of the instantaneous value of the output voltage Vout, the average value Vavg, the set voltage Vset (dependent on the reference voltage Vo), and the on and off state of the switching element in the case where the input voltage is low.
In the above-described ripple converter, when the instantaneous value of the output voltage Vout exceeds the set voltage Vset, the switching element is turned off. The time from the point where the instantaneous value of the output voltage Vout exceeds the set voltage Vset to the point where the switching element is turned off, that is, the time (t1 in the drawing) from the point where the instantaneous value of the output voltage Vout exceeds the set voltage Vset to the point where the instantaneous value becomes the maximum is decided by the circuit construction of the ripple converter, and not decided by the input voltage Vin, and accordingly, the time is basically the same.
Therefore, as shown in FIG. 14, when the input voltage Vin is high, since the slope of the increase in the output voltage Vout becomes steep, the maximum value of the output voltage Vout becomes high in accordance with the input voltage Vin. Here, since the rate at which the output voltage decreases is constant regardless of the input voltage, the time during which the instantaneous value of the output voltage Vo decreases from the maximum to the set voltage Vset becomes longer as the input voltage Vin increases.
On the other hand, the time from the point where the instantaneous value of the output voltage Vout goes below the set voltage Vset to the point where the instantaneous value becomes the minimum, that is, the time (t2 in the drawing) from the point where the instantaneous value of the output voltage Vout goes below the set voltage Vset to the point where the switching element is turned on is the same (does not change) regardless of the input voltage Vin. Accordingly, the minimum value of the output voltage Vout is constant regardless of the input voltage Vin. Moreover, the time from the point where the instantaneous value of the output voltage Vout becomes the minimum to the point where the instantaneous value returns to the set voltage Vset becomes shorter as the input voltage Vin increases because of the increasing amount of change of the voltage. Accordingly, the time during which the switching element is in the on state becomes shorter than the total of the time during which the switching element is in the on state and the time during which the switching element is in the off state. That is, the duty factor of the switching element becomes smaller. In this way, when the duty factor of the switching element becomes smaller, although the off time of the switching element, that is, the time during which the switching element is in the off state becomes longer, since the time during which the instantaneous value of the output voltage Vout is lower than the set voltage Vset in the off time is constant regardless of the input voltage Vin. Accordingly, when the input voltage Vin increases, as shown in FIG. 14, the time during which the instantaneous value of the output voltage Vout is higher than the set voltage Vset becomes longer. Because of this, the output voltage average value Vavg which is the time-average of the instantaneous value of the output voltage Vout becomes higher than the set voltage Vset.
Furthermore, as shown in FIG. 15, when the input voltage Vin is low, after the switching element has been turned on, a voltage transmitted to the output terminal Vout is also reduced in accordance with the input voltage Vin. Here, since the rate at which the output voltage Vout is lowered is constant regardless of the input voltage Vin, the time during which the instantaneous value of the output voltage Vout decreases from the maximum value to the set voltage Vset is more reduced as the input voltage Vin decreases.
On the other hand, the time from the point where the instantaneous value of the output voltage Vout goes below the set voltage Vset to the point where the instantaneous value becomes the minimum value, that is, the time (t2 in the drawing) from the point where the instantaneous value of the output voltage Vout goes below the set voltage Vset to the point where the switching element is turned on is also the same regardless of the input voltage Vin. Moreover, the time from the point where the instantaneous value of the output voltage Vout becomes the minimum to the point where the instantaneous value returns to the set voltage Vset becomes longer because the amount of change of the voltage is more reduced as the input voltage Vin decreases. Accordingly, the time during which the switching element is in the on state becomes longer to the total of the time during which the switching element is in the on state and the time during which the switching element is in the off state. That is, the duty factor of the switching element becomes larger. In this way, when the duty factor of the switching element becomes large, the on time of the switching element becomes long. However, in the on time, since the time during which the instantaneous value of the output voltage Vout is higher than the set voltage Vset is constant regardless of the input voltage Vin, when the input voltage Vin decreases, as shown in FIG. 15, the time during which the instantaneous value of the output voltage Vout is lower than the set voltage Vset becomes longer. Therefore, the output voltage average value Vavg, which is the time average of the instantaneous value of the output voltage Vout, becomes lower than the set voltage Vset.
Thus, when the related ripple converter is used, the average value of the output voltage Vout varies dependent on the value of the switching duty factor.
Furthermore, when ripple converters of the above-described construction are operated in parallel, the output terminals of the ripple converters in parallel operation are connected in parallel. But, as described above, the output voltage of each ripple converter is not constant due to the input voltage value and other factors. Accordingly, there is a possibility that a ripple converter having a high output voltage may adversely affect the operation of other converters. However, since the ripple converters do not contain a current detection mechanism, it is difficult to make uniform the load current of each ripple converter in parallel operation. Therefore, it is difficult to perform stable parallel operation of the ripple converters.